Apple Prores White Paper December 2013 White Paper 2 Apple Prores
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Apple ProRes White Paper December 2013 White Paper 2 Apple ProRes Contents 3 Introduction 4 Authorized Apple ProRes Implementations 5 Apple ProRes Family Overview 7 Properties of Digital Images Frame Size (Full-Width Versus Partial-Width) Chroma Sampling Sample Bit Depth 10 Properties of Apple ProRes Codecs Data Rate Quality Performance Apple ProRes 4444 Alpha Channel Support 20 Appendix Target Data Rates 22 Glossary White Paper 3 Apple ProRes Introduction Apple ProRes is one of the most popular codecs in professional post-production. The Apple ProRes family of video codecs has made it both possible and affordable to edit full-frame, 10-bit, 4:2:2 high-definition (HD), 2K, 4K, and 5K video sources with multistream performance in Final Cut Pro X. This white paper provides in-depth information about all five members of the Apple ProRes family, including technical specifications and performance metrics. White Paper 4 Apple ProRes Authorized Apple ProRes Implementations Apple ProRes is a codec technology developed for high-quality, high-performance editing in Final Cut Pro X. Apple has licensed Apple ProRes to select companies for use in specific products and workflows. In some instances, unauthorized codec implementations have been used in third-party software and hardware products. Using any unauthorized implementation (like the FFmpeg and derivative implementations) may lead to decoding errors, performance degradation, incompatibility, and instability. For a list of all authorized Apple ProRes licensees and developers, and for licensing information, go to support.apple.com/kb/HT5959. If you are using or considering purchasing a product that encodes or decodes Apple ProRes, and that product is not on this list, please contact Apple at [email protected]. White Paper 5 Apple ProRes Apple ProRes Family Overview All members of the Apple ProRes family provide an unparalleled combination of multistream, real-time editing performance coupled with impressive image quality at reduced storage rates. Additionally, all five codecs preserve standard-definition (SD), HD, 2K, 4K, and 5K frame sizes at full resolution. As a variable bit rate (VBR) codec technology, Apple ProRes uses fewer bits on simple frames that would not benefit from encoding at a higher data rate. All Apple ProRes codecs are frame-independent (or “intra-frame”) codecs, meaning that each frame is encoded and decoded independently of any other frame. This technique provides the greatest editing performance and flexibility. • Apple ProRes 4444: The Apple ProRes 4444 codec preserves motion image sequences originating in either 4:4:4 RGB or 4:4:4 Y’CBCR color spaces. With a remarkably low data rate (as compared to uncompressed 4:4:4 HD), Apple ProRes 4444 supports 12-bit pixel depth with an optional, mathematically lossless alpha channel for true 4:4:4:4 support. Apple ProRes 4444 preserves visual quality at the same high level as Apple ProRes 422 (HQ), but for 4:4:4 image sources, which can carry the highest possible color detail. Apple ProRes 4444 is ideal for next-generation video and the most pristine computer graphics sources for cinema-quality compositing. For projects that previously required the Animation codec, Apple ProRes 4444 is a modern replacement with real-time playback support in Final Cut Pro X. Because color detail is preserved, Apple ProRes 4444 is ideal for color grading with 4:4:4 image sources. • Apple ProRes 422 (HQ): Apple ProRes 422 (HQ) offers visually lossless preservation of the highest-quality professional HD video that a (single-link) HD-SDI signal can carry. For this reason, Apple ProRes 422 (HQ) has enjoyed widespread adoption across the video post-production industry. This codec supports full-width, 4:2:2 video sources at 10-bit pixel depths, while retaining its visually lossless characteristic through many generations of decoding and re-encoding. Apple ProRes 422 (HQ) can be used both as an intermediate codec to accelerate workflows for complex, compressed video sources and as an affordable, high-performance alternative to uncompressed 4:2:2 video. White Paper 6 Apple ProRes • Apple ProRes 422: Apple ProRes 422 offers nearly all the benefits of its big brother, Apple ProRes 422 (HQ), but at a significantly lower data rate. It provides visually lossless coding performance for the same full-width, 10-bit, 4:2:2 sequences as Apple ProRes 422 (HQ) with even better multistream real-time editing performance. • Apple ProRes 422 (LT): Like Apple ProRes 422 (HQ) and Apple ProRes 422, the Apple ProRes 422 (LT) codec supports full-width, 10-bit video sequences, but at a target data rate even lower than that of its siblings. Apple ProRes 422 (LT) weighs in at 100 Mbps or less, depending on the particular video format. Apple ProRes 422 (LT) balances incredible image quality with small file sizes, and is perfect for digital broadcast environments where storage capacity and bandwidth are often at a premium. Apple ProRes 422 (LT) is ideal for live multicamera and on-location productions where large amounts of footage are acquired to disk. • Apple ProRes 422 (Proxy): The lowest-bandwidth member of the Apple ProRes family is Apple ProRes 422 (Proxy). This codec maintains HD data rates below 36 Mbps, yet like its higher-rate Apple ProRes 422 siblings, it supports full-frame, 10-bit, 4:2:2 video. Apple ProRes 422 (Proxy) is intended for draft-mode or preview uses where low data rates are required, yet full-resolution video is desired. It is the ideal format for use in offline editing workflows on notebook computers or other systems where storage is at a premium and you have a large quantity of source media. Today’s HD workflows demand offline video formats that support native frame size and aspect ratio. Apple ProRes 422 (Proxy) supports full 1920 x 1080 and 1280 x 720 resolutions, enabling full HD resolution while editing, and accurate representation of Final Cut Pro X motion effects from the creative stages through finishing. White Paper 7 Apple ProRes Properties of Digital Images The technical properties of digital images correspond to different aspects of image quality. For example, high-resolution HD images can carry more detail than their lower-resolution SD counterparts. 10-bit images can carry finer gradations of color, thereby avoiding the banding artifacts that can occur in 8-bit images. The role of a codec is to preserve image quality as much as possible at a particular reduced data rate, while delivering the fastest encoding and decoding speed. The Apple ProRes family supports the three key properties of digital images that contribute to image quality—frame size, chroma sampling, and sample bit depth— while offering industry-leading performance and quality at each supported data rate. In order to appreciate the benefits of the Apple ProRes family as a whole and to choose which family members to use in various post-production workflows, it is important to understand these three properties. Frame Size (Full-Width Versus Partial-Width) Many video camcorders encode and store video frames at less than the full HD widths of 1920 pixels or 1280 pixels, for 1080-line or 720-line HD formats, respectively. When such formats are displayed, they are upsampled horizontally to full HD widths, but they cannot carry the amount of detail possible with full-width HD formats. All Apple ProRes family members can encode full-width HD video sources (sometimes called “full-raster” video sources) to preserve the maximum possible detail an HD signal can carry. Apple ProRes codecs can also encode partial-width HD sources if desired, thereby averting potential quality and performance degradation that results from upscaling partial-width formats prior to encoding. White Paper 8 Apple ProRes Chroma Sampling Color images require three channels of information. In computer graphics, a pixel’s color is typically defined by R, G, and B values. In traditional digital video, a pixel is represented by Y’, CB, and CR values, where Y’ is the “luma” or grayscale value and CB and CR contain the “chroma” or color-difference information. Because the eye is less sensitive to fine chroma detail, it is possible to average together and encode fewer CB and CR samples with little visible quality loss for casual viewing. This technique, known as chroma subsampling, has been used widely to reduce the data rate of video signals. However, excessive chroma subsampling can degrade quality for compositing, color correction, and other image-processing operations. The Apple ProRes family handles today’s popular chroma formats as follows: • 4:4:4 is the highest-quality format for preserving chroma detail. In 4:4:4 image sources, there is no subsampling, or averaging, of chroma information. There are three unique samples, either Y’, CB, and CR or R, G, and B, for every pixel location. Apple ProRes 4444 fully supports 4:4:4 image sources, from either RGB or Y’CBCR color spaces. The fourth “4” means that Apple ProRes 4444 can also carry a unique alpha-channel sample for every pixel location. Apple ProRes 4444 is intended to support 4:4:4:4 RGB+Alpha sources exported from computer graphics applications such as Motion, as well as 4:4:4 video sources from high-end devices such as dual-link HDCAM-SR. • 4:2:2 is considered a high-quality, professional video format in which the chroma values of Y’CBCR images are averaged together such that there is one CB and one CR sample, or one “CB/CR chroma pair,” for each Y’ (luma) sample. This minimal chroma subsampling has traditionally been considered adequate for high-quality compositing and color correction, although better results can be achieved using 4:4:4 sources. 4:2:2 sources are generated by many popular higher-end video camcorder formats, including DVCPRO HD, AVC-Intra/100, and XDCAM HD422/50.